nRF7002 absolute max RF input (−10 dBm) with shared antenna and external BLE FEM

[Göran Nordahl]

Hi,

We are designing a module with nRF54LM20 + nRF7002 (WLCSP) and want to share the 2.4 GHz antenna between Wi-Fi and BLE, following your coexistence guidance (shared antenna mode, PTA, external SPDT controlled by SW_CTRL0, 2.4/5 GHz diplexer to a dual-band antenna).

The BLE path uses an external FEM (Skyworks SKY66114-11) with up to +22 dBm output. The SPDT (Infineon BGS12WN6) has 37 dB typical / 31 dB minimum isolation at 2.4 GHz. When the FEM transmits, the nRF7002 2.4 GHz RF port will therefore see roughly −15 dBm typical and up to about −9 dBm worst case.

The nRF7002 Product Specification v1.2 (Table 12, Absolute maximum ratings) lists “RF input level: −10 dBm”.

Our questions:

1. Does the −10 dBm absolute maximum apply in all nRF7002 states: powered off (VBAT off), sleep/shutdown, idle, and active RX/TX? Or is the 2.4 GHz port more robust when the device is not actively receiving?
2. Is the limit specified for CW, and is there a higher allowable level for short, duty-cycled bursts (BLE packets of a few hundred µs)?
3. When nRF7002 is not active or has granted the antenna to the short-range radio, what does its 2.4 GHz port look like (impedance, internal T/R switch state, any internal protection)?
4. Do you have a recommended architecture or reference design for a shared antenna where the short-range radio uses an external FEM at +20 to +22 dBm? What total isolation toward the nRF7002 RF port do you recommend?
5. If extra protection is needed, what do you recommend: a shunt switch to ground at the nRF7002 2.4 GHz port, two switches in series, a limiter, or an attenuator? What added insertion loss is acceptable for Wi-Fi?
6. We also have an LR1120 transmitting up to +12 dBm at 2.4 GHz on a separate antenna. What antenna-to-antenna isolation do you recommend toward the nRF7002 when it is idle or off?
7. Do the limits differ between the QFN and WLCSP variants?

[Marte Myrvold]

Hi,

I have started looking into this and will get back to you early next week.

Best regards,
Marte

[Marte Myrvold]

Hi,

I asked internally about this and have now received a response.

1. At -10 dBm input power, the active RX reliability is marginal. The device becomes more robust when the internal T/R switch is ON (the input RF signal is tied to ground). When RX is off, the T/R switch is ON.  
2. The -10 dBm limit is specified for CW. Short duty-cycle bursts can extend lifetime, but it is not a linear relationship; it requires a full packet signal with the TSMC model to check.
3. Yes, the internal T/R switch can be turned on to enable the protection. When the nRF7002 is OFF, the 2.4 GHz Wi-Fi port appears inductive (0 ohm at DC and low inductance at 2.4 GHz).
4. The setup of the PTA-control switch is a good approach, in which Bluetooth LE and Wi-Fi are operated on a time-division basis. Since Wi-Fi should be OFF when BLE is ON, a -9 dBm leakage is perfectly fine.
5. They recommend the minimum loss when Wi-Fi is operating. A shunt switch to ground sounds good.
6. When the nRF7002 Wi-Fi RX is OFF, the antenna isolation can be 0 dB. +12 dBm should be OK for the nRF7002 (RX OFF).   
7. QFN has almost the same loss, so the limits are the same.

Best regards,
Marte